Here is the full list of all 18 Dungeon Defenders II achievements.
On four heroes, reach level 25.
Deal 5,000,000 damage with Lightning Strikes Auras.
Reach level 25 on a hero.
Defeat 50 Witherbeasts before they finish burrowing.
Deal 5,000,000 damage with Explosive Traps.
Defeat 100 Kobolds before they light their fuse.
Activate 100 environmental traps.
Deal 5,000,000 damage with Flamethrower Towers.
Reach level 15 on a hero.
Deal 5,000,000 damage with Cannonball Towers.
Reach level 5 on a hero.
Defeat 1,000 special enemies.